1. Field of the Invention
The present invention relates to a misfire detecting circuit for an internal combustion engine for detecting a misfire by detecting an ion current in the combustion chamber of a internal combustion engine.
2. Description of the Related Art
In internal combustion engines, a mixture gas of fuel and air is compressed, and a mixture gas is combusted by a spark generated by applying the high voltage to an ignition plug disposed in the combustion chamber. A state in which the mixture gas is not combusted is called a misfire. When the internal combustion engine is misfiring, output performance is reduced and a mixture gas containing a large amount of fuel flows into an exhaust system. Fuel in the exhaust system causes a problem, for example, in that an exhaust silencer is corroded. Therefore, it is necessary to detect a misfire state and to warn the operator.
As a misfire detection apparatus, there is an apparatus for detecting a misfire by detecting ion current in the combustion chamber. When combustion is performed in the combustion chamber, molecules in the combustion chamber are ionized as a result of the combustion. When a voltage is applied to the inside of the ionized combustion chamber through an ignition plug, a very small current flows, which is called ion current. Since the ion current becomes exceedingly small at the time of misfire, it is possible to detect this ion current and to determine whether a misfire has occurred. The present invention is concerned with a misfire detecting circuit for an internal combustion engine for detecting a misfire by detecting the ion current.
FIG. 20 illustrates a conventional misfire detecting circuit disclosed in, for example, Japanese Patent Laid-Open No. 4-191465.
Referring to FIG. 20, reference numeral 1 denotes an ignition coil; reference numerals 1a and 1b denote the primary coil and the secondary coil, respectively; and reference numeral 3 denotes an ignition plug which is connected to the negative polarity side of the secondary coil 1b. The positive polarity of the primary coil 1a is connected to a power source 8, and the negative polarity thereof is connected to the collector of a transistor 2 for storing electric current. The emitter of the transistor 2 is connected to the ground, and the base is controlled by a control apparatus (not shown) for controlling combustion.
Reference numeral 9 denotes a misfire detecting circuit; reference numeral 5 denotes a capacitor connected to the positive polarity; reference numeral 6 denotes a diode 6 connected between the low electrical potential side of the capacitor 5 and the ground, which diode is connected in a direction in which the capacitor 5 side is formed into the anode. Reference numeral 4 denotes a Zener diode which determines the voltage to be charged in the capacitor 5, which diode is connected between the positive polarity of the secondary coil 1b and the ground; and reference numeral 7 denotes a resistor.
In such a circuit constructed as described above, when the internal combustion engine is ignited, the transistor 2 changes its state suddenly from on to off under the control of a control apparatus (not shown) for controlling combustion. At this time, the primary current of the ignition coil 1 decreases sharply, and a high voltage is generated by a counter electromotive force of the coil. On the secondary side of the ignition coil 1, a voltage developed on the primary side develops in such a way that the voltage is amplified in accordance with the coil winding ratio of the primary coil 1a to that of the secondary coil 1b. Therefore, a voltage of approximately -10 KV to -25 KV is resultingly applied to the ignition plug 3.
In the circuit of FIG. 20, a charge sufficient for detecting ion current is stored in the capacitor 5 by using energy at the time of ignition, and ion current is detected immediately after ignition by a voltage supplied from the capacitor 5. The electric current at the time of ignition flows in the direction of the arrow 3a of FIG. 20, causing discharge at the ignition plug 3 and thus the mixture gas in a combustion chamber 30 is ignited. This discharge current charges the capacitor 5 up to the voltage limited by the Zener diode 4.
When the electric current in the direction of the arrow 3a for ignition decreases to zero, the voltage held by the capacitor 5 is applied to the ignition plug 3. At this time, when combustion is normally performed in the combustion chamber 30, the ion current flows in the direction of the arrow 3b. Since the electric current flowing in the direction of the arrow 3b flows through the resistor 7, a voltage drop is caused. By using this voltage drop as a detection signal, the presence or absence of a misfire is determined based on this voltage drop. That is, since no ion current flows in the case of a misfire, a voltage caused by this ion current does not develop in the output.
Another examples of such a misfire detecting circuit for an internal combustion engine are disclosed in Japanese Patent Laid-Open Nos. 4-265474 and 4-262070. However, these misfire detecting circuits have the problems which will be described below.